Letters From Heaven

The Midrash (בראשית רבה ב, ו) comments on the verse "and they mourned for Ya'akov 70 days - these are the days between the letters."
The commentaries ad loc explain this as a reference to the time span from the letter of annihilation sent out by Haman (of the story of Purim) on the 13 of Nissan until the letter rescinding the decree sent by Mordechai 70 days letter on 23 Sivan.
The question arises, what is the connection between the weeping of the Egyptians to the duration of the decree against the Jews of Persia?
In light of yesterday's post about "Pa'aroh the G-d Lover," we can understand this Midrash entirely different: It is known (שמות רבה ט, יב) that every plague in Egypt lasted three weeks with a week of respite in between, thus amounting to 70 days of respite for all ten plagues. Now, in light of yesterday's post that coins the plagues as "letters," we can understand the words of "between the letters" to mean "between each plague."
יערות דבש ח"ב דרוש ב
See here